The present invention relates to cellular telephone systems, and particularly to base station transmit power control during mobile radio handover in a code division multiple access cellular telephone system.
It is well-known that cellular communications system include base stations arranged geographically in a cellular structure to allow mobile radio stations to communicate with one another and with associated switching systems. Communications occur through the base stations connected in groups to a switching node. The switching node communicates through a gateway to other communications systems.
In a Code Division Multiple Access (CDMA) mobile communications system, mobile radios can communicate with a common base station using the same radio frequency band because each mobile station employs a different modulation code that distinguishes its communications from those of other mobile radios. In addition, spread spectrum communications permit mobile radio transmissions to be simultaneously processed by more than one base station. This gives rise to diversity handovers in which a mobile radio is handed over from one base station to a second base station, all the while maintaining communication with a third base station. The constant communication into the third base station (usually combined with the communication from either the first or second base station) during handover between the first and second insures handover operation without any perceptible disturbance in the voice or data communications. During diversity handover, signals from the active base stations are combined in a common point with decisions being made on the xe2x80x9cqualityxe2x80x9d of the received data. Typically, though not necessarily, this common point is located at the switching node connected to the base stations. At the mobile station, signals from the active base stations are combined.
In soft handover, if a mobile station travels to the edge of an active cell, an adjacent cell""s base station assigns a transceiver to the call while the active base station continues to handle the call. At that point, both the active and the adjacent cell base stations handle the call until a make-before-break condition occurs. Thus, in a soft diversity handover, both base stations handle the call until the mobile station moves sufficiently close to one of the base stations which then exclusively handles the call.
Unfortunately, interference caused by a mobile station""s communication to a common base station can be problematic. Mobile radios which are very close to a common base station transmit information to the base station using the same frequency band at the same time as mobile radios that may be further away. Other factors (such as environmental factors, etc.) may also affect signal strength disparities between multiple mobile radios communicating with a common base station. As a result, some mobile radio communications with a communication base station having the potential to dominate others on the same radio frequency band.
Further, if the transmission power level is the same from all base stations in a soft handover, then the same TPC command is sent towards all base stations involved in the soft handover. Due to errors in the received TPC commands (the TPC commands have no extra coding to protect them from transmission errors) the transmitted power level at each base station can drift even if they were set equal from the beginning.
In an ideal system, each mobile radio sends signals to a common base station at a power level that ensures all of the different mobile-transmitted signals arrive at the base station with about the same average power. Because the environment of the mobile station, the proximity of the mobile stations, etc. are changing, power levels for all radios is vigilantly controlled. As it turns out then, uplink (or reverse) transmit power control (TPC) from the mobile station to the base station is a significant factor in improving the performance and capacity of a CDMA system. Several methods are known for controlling uplink power including control based on a signal-to-noise ratio (or other suitable measure) of signals received from a base station (open loop transmit power control). Another method is for the base station to send transmit power control messages to the mobile station (closed loop power control) based on comparative signal strength received at the base station.
Downlink (or forward) transmit power control is also controlled. In downlink control, the base station varies its transmit power relying in part on power control commands sent by a mobile station to the base station. This procedure allows the mobile radio to ensure its receipt of good quality, non-interfering signals from the base station. For example if base station transmission power is too high (or unnecessarily high), the transmission may interfere with other communications with other radios. On the other hand, if base station power is too low (to minimize interference) the mobile station may receive too poor a quality of signal. Download control balances those issues.
Many different protocols for downlink power control are known. U.S. Pat. No. 5,345,598 (commonly assigned), for example, describes a single control loop for downlink and uplink power control.
The present invention addresses downlink power control from the base station to the mobile station in the context of soft or softer handover. A conventional approach to downlink transmission power control is for all base stations involved in the soft handover to transmit to the mobile station at the same downlink transmission power level. However, this results in increased interference levels which ultimately reduce the capacity of the system. Moreover, because of errors in the transmit power command ultimately received at each base station involved in the soft handover, the transmit power level at each base station can xe2x80x9cdriftxe2x80x9d apart even though the transmitted power level was initially set to be equal for all of those base stations.
The present invention controls the downlink power level at one (or more) of the base stations involved in the soft handover so that the ratio between the downlink transmit powers from all of the involved base stations to the mobile station is equal to the ratio of the received signal strengths from the mobile stations at each of the involved base stations. Alternatively, the downlink power levels are controlled by a comparison based on the received signal strength to received interference for the connection to a specific mobile station. In the latter case, adjustments are made only on the downlink transmission power level to the specific mobile station.